Love Undertaker to death, think he's one of the greatest of all time and his longevity in the business is astounding.
That said, he wasn't always as revered. There was a period of time in Undertaker's career around roughly 2000-2004 where he was out of shape and not putting on great matches, and his character work wasn't the greatest, either.
It's a short blip on a phenomenal career, but with time people sometimes forget that Undertaker wasn't always as revered as he is today. He was never *the* guy in the company. He was behind Hogan/Warrior, then behind Hart/HBK, then behind Austin/Rock, then behind Cena/Batista and eventually just became a WM attraction. He'll always be one of the greatest, IMO his character is the greatest of all time in terms of pure gimmicks, but he's never been more popular than wrestlers like Hogan, Flair, Hart, Michaels, Rock, Austin, and Cena during their respective primes.